In electron optics, as a rule, groups of several controllable elements, which act on the electron beam, are used to vary the imaging conditions. For example, it is known to use a two-lens or a three-lens imaging system (condenser system) in electron microscopes at the illuminating end so that the object area which can be illuminated is variable when the positions of the electron source and object plane are fixed. Likewise, it is known for a fixed object plane and a fixed projection plane that the imaging scale is variable with the aid of a three-lens imaging system at the imaging end without a rotation of the image occurring when there is a variation in magnification.
The dependence of the imaging characteristics on the applied lens excitation can be very precisely computed for individual electron-optical imaging elements. However, for the combination of several imaging elements, mechanical tolerances and, with magnetic elements, remanences in the iron circuit of these magnetic elements, cause too great a deviation of the actual imaging characteristics (especially with respect to focusing, maintaining focus or image rotation) from the theoretically computed ones. For this reason, electron microscopes obtained in the marketplace up to now are therefore experimentally calibrated when first taken into use. That is, the excitations of the individual elements corresponding to various operating conditions are determined by manual adjustment and thereafter stored. In later use of the electron microscope, the excitation values which correspond to the selected operating condition are then read out of the memory and are applied to the current sources or voltage sources of the imaging elements. The very substantial calibration effort therefore limits the number of selectable operating conditions, i.e. alignment effort. The operation of the electron microscope is then limited to the experimentally calibrated discrete operating conditions.
U.S. Pat. No. 4,871,912 discloses an electron microscope wherein operating conditions are also adjustable which were not previously calibrated. For this purpose, an external computer is provided which is in addition to the internal computer usually provided in the operator console of the electron microscope. If, for example, a magnification value is inputted via the keyboard of the external computer which lies between two calibrated magnification steps, then the parameter values of the imaging system corresponding to the two mutually adjacent calibrated magnification steps are read into the external computer; parameters are calculated which correspond approximately to the inputted magnification by means of linear interpolation between the calibrated parameter values; and, the electron microscope is then driven in correspondence to these parameter values.
The dependency of the focal width on the lens current is nonlinear for magnetic lenses and is, as a rule, different for each lens because of remanences and manufacturing tolerances. For this reason, an adjustment of adequate accuracy of the imaging conditions for intermediate values is not ensured. In practice, defocused images occur in the final image plane. Furthermore, the input of the intermediate values via the external computer takes a great deal of time and is impractical. This input does not permit, for example, varying the magnification continuously or in fine steps preselectable by the operator while at the same time viewing the object image in the projection plane. However, this last-mentioned capability is required when the optimal magnification is to be adjusted for a detail of the object of interest.
An interpolation between calibrated parameter values is also referred to in U.S. Pat. No. 4,851,674 in connection with a correction of astigmatism or deflection in an electron microscope. However, here no specific data are obtained with respect to the interpolation.